Silicon carbon material and preparation method thereof, secondary battery and electronic device

Abstract

The invention provides a silicon-carbon material and a preparation method thereof, a secondary battery and an electronic device, the silicon-carbon material comprises a carbon skeleton and silicon particles, the carbon skeleton comprises amorphous carbon and carbon nanotubes, in a thermogravimetric analysis curve of the silicon-carbon material, a first thermal weight loss peak exists in a temperature interval of 500-800 DEG C, and a second thermal weight loss peak exists in a temperature interval of 500-800 DEG C; a second thermal weight loss peak exists in the temperature range of 800-1000 DEG C, the ratio of the intensity of the first thermal weight loss peak to the intensity of the second thermal weight loss peak is R, and R is larger than or equal to 10 and smaller than or equal to 1000. The silicon-carbon material provided by the invention can improve the cycle performance and the rate capability of the secondary battery.

Description

Technical Field

[0001] The present application relates to the field of electrochemical energy storage, and in particular to a silicon-carbon material, a preparation method of the silicon-carbon material, a secondary battery using the silicon-carbon material, and an electronic device using the secondary battery. Background Art

[0002] Lithium-ion batteries have the advantages of high energy density, high working voltage, good safety, and environmental friendliness, and are widely used in consumer electronics fields such as mobile phones and laptop computers. Silicon-carbon materials are regarded as an ideal choice for the next-generation anode materials of lithium-ion batteries due to their high theoretical specific capacity (about 4200 mAh / g). However, in practical applications, silicon-carbon materials still face two major challenges: poor rate performance and poor structural stability. The poor rate performance is mainly due to the slow lithium-ion diffusion kinetics in the silicon-carbon material during charge and discharge, resulting in a rapid capacity decay at high current densities. In addition, the silicon-carbon material will undergo significant volume expansion (up to more than 300%) during charge and discharge, leading to particle fragmentation, active material shedding, and electrode structure damage, thus seriously affecting the cycle stability and service life of the battery. Summary of the Invention

[0003] The present application provides a silicon-carbon material, a preparation method thereof, a secondary battery, and an electronic device.

[0004] In the first aspect of the present application, a silicon-carbon material is provided, which includes a carbon skeleton and silicon particles. The carbon skeleton includes amorphous carbon and carbon nanotubes. In the thermogravimetric analysis curve of the silicon-carbon material, there is a first thermal weight loss peak in the temperature range of 500°C to 800°C, and a second thermal weight loss peak in the temperature range of 800°C to 1000°C. The ratio of the intensity of the first thermal weight loss peak to the intensity of the second thermal weight loss peak is R, and 10 ≤ R ≤ 1000.

[0005] In the present application, the mass ratios of the porous carbon and the carbon nanotubes in the silicon-carbon composite material can be obtained through these two weight loss peaks. The inventor found that when the ratio of the intensities of the two weight loss peaks is within the above range, it indicates that the silicon-carbon material contains an appropriate content of carbon nanotubes. The silicon-carbon material has good electrical conductivity and structural stability, which is beneficial to improving the specific capacity and rate performance of the silicon-carbon material, and improving the cycle performance and energy density of the secondary battery. And within the above appropriate range, the contents of the amorphous carbon and the carbon nanotubes can also be designed, so that both the amorphous carbon and the carbon nanotubes maintain appropriate contents to further improve the cycle performance and rate performance of the silicon-carbon material.

[0006] Based on the first aspect, in some embodiments, 15 ≤ R ≤ 850, which is beneficial to further improving the cycle performance and rate performance of the silicon-carbon material.

[0007] Based on the first aspect, in some embodiments, the diameter ratio of the carbon nanotubes is L, where 1.2 ≤ L ≤ 2. This can effectively improve the electrical conductivity and mechanical properties of the silicon-carbon material, while also facilitating the maintenance of the structural stability and cycling performance of the silicon-carbon material. Moreover, when the diameter ratio of the carbon nanotubes is within the above range, it is beneficial for the silicon-carbon material to have an appropriate specific surface area, improve the dispersibility and uniformity of the carbon nanotubes, enhance the mechanical properties and stability of the silicon-carbon material, and improve the overall electrical conductivity and electrochemical performance of the silicon-carbon material.

[0008] Based on the first aspect, in some embodiments, the average roundness of the silicon-carbon material is from 0.5 to 0.95. When the silicon-carbon material is applied to the negative electrode sheet, the above roundness range will improve the contact between the silicon-carbon material and the conductive agent in the negative electrode sheet, facilitate the improvement of electron transfer, contribute to the improvement of the electrical conductivity, mechanical strength and cycling stability of the silicon-carbon material, and thus improve the cycling performance and rate performance of the silicon-carbon material.

[0009] Based on the first aspect, in some embodiments, the powder conductivity of the silicon-carbon material is from 0.1 S / cm to 10 S / cm. The silicon-carbon material has good electrical conductivity, improves the charge and discharge efficiency of the secondary battery, reduces capacity attenuation, improves the cycle life and rate performance of the secondary battery; and it is also beneficial to maintain an appropriate current during the charge and discharge process of the secondary battery, improve the thermal stability of the silicon-carbon material, and reduce the probability of thermal runaway or instability of the secondary battery.

[0010] Based on the first aspect, in some embodiments, the elastic modulus of the silicon-carbon material is from 50 GPa to 200 GPa. This is beneficial for the silicon-carbon material to maintain good structural stability during the charge and discharge process, reduce the possibility of reducing the cycling stability of the silicon-carbon material due to cracks generated by volume changes of the silicon-carbon material particles during the charge and discharge process. The silicon-carbon material has a certain pressure-bearing capacity, reduces the damage of the silicon-carbon material during the cold pressing process, improves the mechanical strength of the silicon-carbon material, and improves the cycling performance and rate performance of the secondary battery.

[0011] Based on the first aspect, in some embodiments, the cross-section of the silicon-carbon material includes a first test area and a second test area with an area of 1000 nm 2 . The first test area and the second test area do not overlap. Based on the area of the first test area, the area ratio of the carbon nanotubes is S1%, and based on the area of the second test area, the area ratio of the carbon nanotubes is S2%. |S1 - S2| ≤ 0.067. This is beneficial for the uniform distribution of the carbon nanotubes in the silicon-carbon material, reduces the phenomenon that the carbon nanotubes may aggregate in some regions of the carbon skeleton while being sparse in other regions, thereby improving the electrochemical performance and mechanical strength of the silicon-carbon material, and improving the cycling performance and rate performance of the secondary battery.

[0012] The second aspect of the present application provides a method for preparing a silicon-carbon material, including the following steps: S1. Mix carbon nanotubes, formaldehyde, phenol and an alkaline catalyst, and stir to obtain a suspension, wherein the mass ratio of carbon nanotubes to phenol is (0.03 - 3):100; S2. Filter the suspension by suction, and obtain a solid product after drying; S3. Carbonize and activate the solid product in sequence to obtain porous carbon; S4. Use chemical vapor deposition to perform silicon deposition and carbon coating on the porous carbon to obtain a silicon-carbon material. In the preparation method provided by the present application, carbon nanotubes, formaldehyde, phenol and ammonia water are mixed to obtain a suspension, and by controlling the mass ratio of carbon nanotubes to phenol, in the initial stage of the reaction for forming amorphous carbon, the carbon nanotubes are uniformly mixed with the reactants, so as to facilitate the uniform distribution of carbon nanotubes in the amorphous carbon, and make the silicon-carbon material contain an appropriate content of carbon nanotubes and amorphous carbon, thereby improving the cycle performance and rate performance of the silicon-carbon material.

[0013] The third aspect of the present application provides a secondary battery, including a negative electrode sheet, a positive electrode sheet and an electrolyte. The negative electrode sheet includes the silicon-carbon material described above or the silicon-carbon material obtained by the preparation method described above. The secondary battery containing the silicon-carbon material can improve the cycle performance and rate performance of the secondary battery.

[0014] Based on the third aspect, in some embodiments, the electrolyte includes ethyl propionate and propyl propionate. Based on the mass of the electrolyte, the mass ratio of ethyl propionate is A1, and the mass ratio of propyl propionate is A2, and 2 ≤ A2 / A1 ≤ 6. This is beneficial to further improve the cycle performance, rate performance and swelling performance of the secondary battery.

[0015] Based on the third aspect, in some embodiments, the electrolyte includes a sulfur-oxygen double bond compound, and the sulfur-oxygen double bond compound is selected from at least one of methylene methane disulfonate, 1,3-propane sultone, 1,3-propane sultone, 2,4-butane sultone or vinylene sulfate. This is beneficial to further improve the cycle performance, rate performance and swelling performance of the secondary battery.

[0016] Based on the third aspect, in some embodiments, based on the mass of the electrolyte, the mass ratio of the sulfur-oxygen double bond compound is P%, and 1 ≤ P ≤ 5. This is beneficial to further improve the cycle performance, rate performance and swelling performance of the secondary battery.

[0017] The fourth aspect of the present application provides an electronic device, including the secondary battery described above. The secondary battery has excellent cycle performance and rate performance, which is beneficial to improving the service life of the electronic device. Detailed embodiments

[0018] The technical solutions in the embodiments of the present application will be described clearly and in detail below. Obviously, the described embodiments are some, but not all, of the embodiments of the present application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which this application belongs. The terms used in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0019] To address the poor rate performance and poor structural stability of silicon-carbon materials, researchers have proposed various modification strategies, including nanosizing, carbon coating, introducing conductive additives, and designing porous structures, etc. However, these methods still have certain limitations in practical applications. For example, nanosizing may lead to an increase in the surface area of the material and an increase in side reactions; carbon coating may limit the diffusion channels of lithium ions and affect the rate performance. Therefore, developing a silicon-carbon material that can both improve the rate performance and enhance the structural stability remains the focus and difficulty of current research. Future research directions may include developing new nanostructures, optimizing the conductivity and stability of the carbon matrix, and exploring new synthesis methods to achieve the efficient preparation and performance improvement of silicon-carbon materials.

[0020] One embodiment of the present application provides a secondary battery, which includes a housing, an electrode assembly, and an electrolyte. The electrode assembly and the electrolyte are both located inside the housing.

[0021] The housing can be a packaging bag obtained by encapsulating with a packaging film (such as an aluminum-plastic film), for example, when the secondary battery is a soft-pack battery. In some other embodiments, the secondary battery can also be a steel-shell battery, an aluminum-shell battery, etc.

[0022] The electrode assembly includes a positive electrode tab, a negative electrode tab, and a separator. The separator is disposed between the positive electrode tab and the negative electrode tab. The electrode assembly can be a stacked structure, which is formed by laminating the positive electrode tab, the separator, and the negative electrode tab. In some other embodiments, the electrode assembly can also be a wound structure, which is formed by laminating and then winding the positive electrode tab, the separator, and the negative electrode tab.

[0023] Negative electrode tab

[0024] The negative electrode tab includes a negative current collector and a negative active layer located on the surface of the negative current collector. The negative current collector includes: copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or any combination thereof, and can also be a composite current collector disclosed in any prior art, such as but not limited to the current collector formed by combining the aforementioned conductive foils and polymer substrates. In the negative electrode tab, the negative active layer includes a silicon-carbon material.

[0025] The silicon-carbon material provided by the present application includes a carbon skeleton and silicon particles. The carbon skeleton includes amorphous carbon and carbon nanotubes. In the thermogravimetric analysis curve of the silicon-carbon material, there is a first thermal weight loss peak in the temperature range of 500 °C to 800 °C, and a second thermal weight loss peak in the temperature range of 800 °C to 1000 °C. The ratio of the intensity of the first thermal weight loss peak to the intensity of the second thermal weight loss peak is R, and 10 ≤ R ≤ 1000.

[0026] In the present application, the first thermal weight loss peak is the peak formed during the decomposition process of amorphous carbon in air, and the second thermal weight loss peak is the peak formed during the decomposition process of carbon nanotubes in air. The mass percentages of porous carbon and carbon nanotubes in the silicon-carbon composite material can be obtained through these two weight loss peaks. The inventors found that when the ratio of the intensities of the two weight loss peaks is within the above range, it indicates that the silicon-carbon material contains an appropriate amount of carbon nanotubes, and the silicon-carbon material has good electrical conductivity and structural stability, which is beneficial to improving the specific capacity and rate performance of the silicon-carbon material, and improving the cycle performance and energy density of the secondary battery. And within the above appropriate range, the contents of amorphous carbon and carbon nanotubes can also be designed, so that both amorphous carbon and carbon nanotubes maintain appropriate contents to further improve the cycle performance and rate performance of the silicon-carbon material.

[0027] If R is small, such as less than 10, the silicon-carbon material contains more carbon nanotubes, which will affect the specific capacity of the silicon-carbon material and reduce the structural stability of the silicon-carbon material, thereby reducing the cycle performance and rate performance of the secondary battery. If R is large, such as greater than 1000, the silicon-carbon material contains less carbon nanotubes, which will reduce the electrical conductivity and structural stability of the silicon-carbon material, and reduce the rate performance and cycle performance of the secondary battery.

[0028] In some embodiments, R can be 10, 20, 50, 100, 200, 300, 500, 600, 700, 800, 900, 1000 or any value within the range composed of any two of the above values.

[0029] In some embodiments, 15 ≤ R ≤ 850 is beneficial to further improving the cycle performance and rate performance of the silicon-carbon material.

[0030] In some embodiments, the diameter ratio of the carbon nanotubes is L, where 1.2 ≤ L ≤ 2. The diameter ratio of the carbon nanotubes can refer to the ratio of the outer diameter to the inner diameter of the carbon nanotubes. When the diameter ratio of the carbon nanotubes is within the above range, it can effectively improve the conductivity and mechanical properties of the silicon-carbon material, and at the same time, it is also beneficial to maintain the structural stability and cycling performance of the silicon-carbon material. And when the diameter ratio of the carbon nanotubes is within the above range, it is beneficial to make the silicon-carbon material have an appropriate specific surface area, improve the dispersibility and uniformity of the carbon nanotubes, improve the mechanical properties and stability of the silicon-carbon material, and improve the overall conductivity and electrochemical performance of the silicon-carbon material. In some embodiments, the diameter ratio L of the carbon nanotubes can be 1.2, 1.3, 1.5, 1.6, 1.8, 2 or any value within the range composed of any two of the above values.

[0031] In some embodiments, the average circularity of the silicon-carbon material is from 0.5 to 0.95. When the circularity of the silicon-carbon material is within the above suitable range, when the silicon-carbon material is applied to the negative electrode sheet, the above circularity range will improve the contact between the silicon-carbon material and the conductive agent in the negative electrode sheet, which is beneficial to improving electron transport, contributing to improving the conductive performance, mechanical strength and cycling stability of the silicon-carbon material, and thus improving the cycling performance and rate performance of the silicon-carbon material. In some embodiments, the average circularity of the silicon-carbon material can be 0.5, 0.6, 0.7, 0.8, 0.9, 0.95 or any value within the range composed of any two of the above values.

[0032] In some embodiments, the powder conductivity of the silicon-carbon material is from 0.1 S / cm to 10 S / cm. When the powder conductivity of the silicon-carbon material is within the above suitable range, it enables the silicon-carbon material to maintain good conductive performance, improves the charge-discharge efficiency of the secondary battery, reduces capacity attenuation, and improves the cycle life and rate performance of the secondary battery; and it is also beneficial to maintain an appropriate current during the charge-discharge process of the secondary battery, improve the thermal stability of the silicon-carbon material, and reduce the risk of thermal runaway or instability of the secondary battery. In some embodiments, the powder conductivity of the silicon-carbon material is 0.1 S / cm, 0.5 S / cm, 1 S / cm, 2 S / cm, 3 S / cm, 4 S / cm, 5 S / cm, 6 S / cm, 7 S / cm, 8 S / cm, 9 S / cm, 10 S / cm or any value within the range composed of any two of the above values.

[0033] In some embodiments, the elastic modulus of the silicon-carbon material is from 50 GPa to 200 GPa. Keeping the elastic modulus of the silicon-carbon material within the above suitable range is conducive to maintaining good structural stability of the silicon-carbon material during charge and discharge, reducing the possibility of reducing the cycle stability of the silicon-carbon material due to cracks generated by volume changes of the silicon-carbon material particles during charge and discharge. The silicon-carbon material has a certain pressure-bearing capacity, reducing the damage of the silicon-carbon material during cold pressing, improving the mechanical strength of the silicon-carbon material, and improving the cycle performance and rate performance of the secondary battery. In some embodiments, the elastic modulus of the silicon-carbon material can be 50 GPa, 60 GPa, 70 GPa, 80 GPa, 90 GPa, 95 GPa, 100 GPa, 110 GPa, 120 GPa, 130 GPa, 140 GPa, 150 GPa, 160 GPa, 170 GPa, 180 GPa, 190 GPa, 200 GPa or any value within the range formed by any two of the above values.

[0034] In some embodiments, the cross-section of the silicon-carbon material includes a first test area with an area of 1000 nm 2 and a second test area with an area of 1000 nm 2 The first test area and the second test area do not overlap. Based on the area of the first test area, the area ratio of the carbon nanotubes is S1%, and based on the area of the second test area, the area ratio of the carbon nanotubes is S2%. |S1 - S2| ≤ 0.067. Using the difference value of |S1 - S2| can reflect the distribution uniformity of the carbon nanotubes in the silicon-carbon material. When the difference value of |S1 - S2| is closer to 0, it indicates that the carbon nanotubes are more uniformly distributed within the particles of the silicon-carbon material. The smaller the difference value of |S1 - S2|, the worse the distribution uniformity of the carbon nanotubes within the particles of the silicon-carbon material. When the difference value of |S1 - S2| is within the above range, the carbon nanotubes are uniformly distributed in the silicon-carbon material, reducing the phenomenon that the carbon nanotubes may aggregate in some areas of the carbon skeleton and be sparse in other areas, thereby improving the electrochemical performance and mechanical strength of the silicon-carbon material, and improving the cycle performance and rate performance of the secondary battery. In some embodiments, the difference value of |S1 - S2| can be 0.01, 0.02, 0.03, 0.04, 0.047, 0.05, 0.06, 0.67 or any value within the range formed by any two of the above values. In some embodiments, |S1 - S2| ≤ 0.047.

[0035] In some embodiments, the electrolyte includes ethyl propionate and propyl propionate. Based on the mass of the electrolyte, the mass fraction of ethyl propionate is A1, and the mass fraction of propyl propionate is A2, where 2 ≤ A2 / A1 ≤ 6. The inclusion of ethyl propionate and propyl propionate in the electrolyte is beneficial to improving the cycling performance, rate performance, and swelling performance of the secondary battery; when A2 / A1 satisfies the above relationship, it is beneficial to further improve the cycling performance, rate performance, and swelling performance of the secondary battery. In some embodiments, the ratio of A2 / A1 can be 2, 3, 4, 5, 6, or any value within the range formed by any two of the above values. Preferably, 3 ≤ A2 / A1 ≤ 5.

[0036] In some embodiments, the electrolyte includes a sulfur-oxygen double bond compound, and the sulfur-oxygen double bond compound is selected from at least one of methylene methane disulfonate, 1,3-propane sultone, 1,3-propane sulfone, 2,4-butane sultone, or vinylene sulfate. The inclusion of the sulfur-oxygen double bond compound in the electrolyte is beneficial to further improving the cycling performance, rate performance, and swelling performance of the secondary battery.

[0037] Based on the mass of the electrolyte, the mass fraction of the sulfur-oxygen double bond compound is P%, where 1 ≤ P ≤ 5. The mass fraction of the sulfur-oxygen double bond compound in the electrolyte within the above range is beneficial to further improving the cycling performance, rate performance, and swelling performance of the secondary battery. In some embodiments, P can be 1, 2, 3, 4, 5, or any value within the range formed by any two of the above values. Preferably, 1.5 ≤ P ≤ 4.5.

[0038] In some embodiments, the negative electrode active layer further includes a binder and a conductive agent. In some embodiments, the binder includes, but is not limited to: polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon, etc.

[0039] In some embodiments, the conductive agent includes, but is not limited to: carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based materials are selected from carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based materials are selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0040] This application provides a method for preparing a silicon-carbon material, including the following steps:

[0041] S1. Mix carbon nanotubes, formaldehyde, phenol, and an alkaline catalyst, and stir to obtain a suspension.

[0042] In the suspension, the mass ratio of carbon nanotubes to phenol is (0.03 - 3):100, and the mass ratio of formaldehyde to phenol is 1:3. The catalyst can be ammonia water, and the mass percentage of ammonia water is 5% to 25%. In some embodiments, the mass ratio of carbon nanotubes to phenol can be 0.03:100, 0.05:100, 0.1:100, 0.3:100, 0.5:100, 1:100, 1.3:100, 1.5:100, 2:100, 2.5:100, 3:100 or any value within the range formed by any two of the above values. The mass percentage of ammonia water can be 5%, 10%, 13%, 15%, 18%, 20%, 23%, 25% or any value within the range formed by any two of the above values. The stirring time can be 2 h, 3 h, 4 h, 5 h or any value within the range formed by any two of the above values.

[0043] Under the action of the catalyst, carbon nanotubes, formaldehyde and phenol react to form phenolic resin containing carbon nanotubes, and the carbon nanotubes are uniformly dispersed in the phenolic resin.

[0044] S2. Perform suction filtration on the suspension, and obtain a solid product after drying.

[0045] Use a vacuum suction filter, wash 3 times with deionized water and then wash 2 times with ethanol, filter to obtain a filtered product, and dry it to obtain a solid product.

[0046] S3. Carbonize and activate the solid product in sequence to obtain porous carbon.

[0047] In this step, the solid product undergoes carbonization and activation stages in sequence. Carbonization stage: Under an inert gas (such as nitrogen or argon), the carbonization temperature is 600°C to 1500°C, and the carbonization time is 2 h to 6 h. Activation stage: Under the atmosphere of a mixed gas of water and carbon dioxide, the activation temperature is 700°C to 1000°C, and the activation time is 2 h to 20 h. In this step, through the above carbonization and activation, it is beneficial to obtain porous carbon with a more stable structure.

[0048] In some embodiments, the carbonization temperature can be 600°C, 800°C, 900°C, 1000°C, 1200°C, 1400°C, 1500°C or any value within the range formed by any two of the above values; the carbonization time can be 2 h, 3 h, 4 h, 5 h, 6 h or any value within the range formed by any two of the above values; the activation temperature can be 700°C, 800°C, 900°C, 1000°C or any value within the range formed by any two of the above values; the activation time can be 2 h, 5 h, 10 h, 15 h, 20 h or any value within the range formed by any two of the above values.

[0049] S4. Silicon deposition and carbon coating are carried out on the porous carbon by chemical vapor deposition to obtain the silicon-carbon material.

[0050] By chemical vapor deposition, in a silane atmosphere, a silicon-containing gas is deposited on the porous carbon containing carbon nanotubes. The deposition temperature is 400°C to 600°C, and the deposition time is 2 h to 20 h. Then, carbon coating is carried out on the porous carbon deposited with silicon particles. In an acetylene atmosphere containing carbon gas, the carbon coating temperature is 500°C to 900°C, and the carbon coating time is 0.1 h to 6 h to further stabilize the porous carbon structure and obtain the silicon-carbon material. In some embodiments, the deposition temperature can be 400°C, 500°C, 600°C, or any value within the range composed of any two of the above values; the deposition time can be 2 h, 5 h, 10 h, 15 h, 20 h, or any value within the range composed of any two of the above values. The carbon coating temperature can be 500°C, 600°C, 700°C, 800°C, 900°C, or any value within the range composed of any two of the above values; the carbon coating time can be 0.1 h, 0.5 h, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, or any value within the range composed of any two of the above values.

[0051] In the preparation method provided by the present application, carbon nanotubes, formaldehyde, phenol, and ammonia water are mixed to obtain a suspension, and by controlling the mass ratio of carbon nanotubes to phenol, the carbon nanotubes are uniformly mixed with the reactants at the initial stage of the reaction to form amorphous carbon, so as to facilitate the uniform distribution of carbon nanotubes in the amorphous carbon and make the silicon-carbon material contain an appropriate content of carbon nanotubes and amorphous carbon, thereby improving the cycle performance and rate performance of the silicon-carbon material.

[0052] Separator

[0053] There are no particular limitations on the material and shape of the separator used in the secondary battery of the present application, and it can be any technology disclosed in the prior art. In some embodiments, the separator includes a polymer or an inorganic substance formed of a material stable to the electrolyte of the present application.

[0054] For example, the separator may include a base material layer and a surface treatment layer. The base material layer is a non-woven fabric, a film, or a composite film having a porous structure, and the material of the base material layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite film can be selected.

[0055] A surface treatment layer is provided on at least one surface of the base material layer. The surface treatment layer can be a polymer layer, an inorganic layer, or a layer formed by mixing polymers and inorganic substances. The inorganic layer includes inorganic particles and a binder. The inorganic particles are selected from at least one of alumina, silica, magnesia, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder is selected from at least one of polyvinylidene fluoride, copolymer of vinylidene fluoride - hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyethylene alkoxide, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer contains a polymer, and the material of the polymer is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyethylene alkoxide, polyvinylidene fluoride, and poly(vinylidene fluoride - hexafluoropropylene).

[0056] Electrolyte

[0057] According to some embodiments of the present application, the electrolyte includes an organic solvent, a lithium salt, and an optional additive. The organic solvent in the electrolyte of the present application can be any organic solvent known in the prior art that can be used as a solvent for the electrolyte. There is no limitation on the electrolyte used in the electrolyte according to the present application, and it can be any electrolyte known in the prior art. The additive of the electrolyte according to the present application can be any additive known in the prior art that can be used as an electrolyte additive. In some embodiments, the organic solvent includes, but is not limited to: ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate, or ethyl propionate. In some embodiments, the organic solvent includes an ether solvent, such as including at least one of 1,3 - dioxolane (DOL) and ethylene glycol dimethyl ether (DME). In some embodiments, the lithium salt includes at least one of an organic lithium salt or an inorganic lithium salt. In some embodiments, the lithium salt includes, but is not limited to: lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ), lithium difluorophosphate (LiPO 2 F 2 ), lithium bis(trifluoromethanesulfonyl)imide LiN(CF 3 SO 2 ) 2 (LiTF S I), lithium bis(fluorosulfonyl)imide Li(N(SO 2 F) 2 )(LiF SI), lithium bis(oxalato)borate LiB(C 2 O 4 ) 2(LiB OB) or lithium difluoro(oxalato)borate LiBF 2 (C 2 O 4 )(LiDFOB). In some embodiments, the additive includes at least one of fluoroethylene carbonate and adiponitrile.

[0058] Positive electrode sheet

[0059] The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer provided on the positive electrode current collector. The positive electrode current collector can be made of aluminum foil, nickel foil, etc., or can be a composite current collector disclosed in any prior art, such as but not limited to the current collector formed by combining the aforementioned conductive foil and polymer substrate. The positive electrode active layer contains a positive electrode active material, and the positive electrode active material includes a compound that can reversibly intercalate and deintercalate lithium ions (i.e., a lithiated intercalation compound). In some embodiments, the positive electrode active material can include a lithium transition metal composite oxide. The lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese, and nickel. In some embodiments, the positive electrode active material can include but not be limited to at least one of lithium cobaltate, lithium nickel manganese cobaltate, lithium nickel manganese aluminate, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel-type lithium manganate, spinel-type lithium nickel manganate, and lithium titanate.

[0060] The positive electrode active layer also contains a binder for bonding the positive electrode active material particles to facilitate the formation of a film layer and at the same time improve the bonding force between the positive electrode active layer and the positive electrode current collector. In some embodiments, the binder can include but not be limited to at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon, etc.

[0061] The positive electrode active layer may also contain a conductive material, and the conductive material includes but not limited to carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, the carbon-based materials can include but not be limited to natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based materials can include but not be limited to metal powder or metal fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer can be a polyphenylene derivative.

[0062] The above secondary battery is applied to an electronic device to supply power to a load in the electronic device. Moreover, the silicon-carbon material in the above secondary battery has excellent cycling performance and swelling performance, which is beneficial to improving the service life of the electronic device. Among them, the electronic device may include but is not limited to laptop computers, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, liquid crystal televisions, portable cleaners, portable CD players, mini discs, transceivers, electronic notebooks, calculators, memory cards, portable tape recorders, radios, backup power supplies, motors, automobiles, motorcycles, motorized bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household storage batteries, and lithium-ion capacitors, etc.

[0063] The present application will be described below through specific examples and comparative examples. Those skilled in the art should understand that the preparation methods described in the present application are only examples, and any other suitable preparation methods are within the scope of the present application.

[0064] Example 1-1

[0065] Preparation of silicon-carbon material:

[0066] Mix 1 g of carbon nanotubes, 55 g of formaldehyde, 165 g of phenol, and 30 g of ammonia water, and stir for 2 hours to form a uniform suspension containing carbon nanotubes. Subsequently, the suspension is subjected to suction filtration, washed 3 times with deionized water and then 2 times with ethanol, and vacuum dried at 80 °C for 24 h to obtain a solid product.

[0067] Then, the dried solid product is carbonized and activated in sequence. Carbonization stage: In an argon atmosphere, the carbonization temperature is 600 °C and the carbonization time is 2 h. Activation stage: In a mixed gas atmosphere of water and carbon dioxide, the activation temperature is 900 °C and the activation time is 4 h, to obtain porous carbon containing carbon nanotubes.

[0068] Finally, through chemical vapor deposition, a silicon-containing gas (such as silane, SiH 4 ) is deposited on the porous carbon containing carbon nanotubes. The deposition temperature is 550 °C and the deposition time is 6 h. Then, carbon-containing gas (such as acetylene, C 2 H 2 ) coating is carried out. The carbon coating temperature is 600 °C and the carbon coating time is 0.5 h to obtain silicon-carbon particulate material.

[0069] Preparation of lithium-ion battery:

[0070] Preparation of the positive electrode: Lithium cobalt oxide, conductive carbon black, and polyvinylidene fluoride (PVDF) were thoroughly stirred and mixed evenly in an N-methylpyrrolidone solvent system according to a weight ratio of 95%:2.5%:2.5% to obtain a positive electrode slurry. The prepared positive electrode slurry was coated on an aluminum foil positive electrode current collector, dried, and cold-pressed to obtain a positive electrode plate.

[0071] Preparation of the negative electrode: Graphite, silicon-carbon material, conductive agent carbon nanotubes, and binder poly(methyl acrylate) were mixed according to a solid mass ratio of 88.4%:9.8%:0.1%:1.7%, and kneaded and dispersed at a solid content of about 38% - 50 wt% to prepare a negative electrode slurry. The prepared negative electrode slurry was coated on a copper foil negative electrode current collector, dried, and cold-pressed to obtain a negative electrode.

[0072] Preparation of the electrolyte: In a dry argon environment, LiPF 6 was added to a mixed solution of 1,3-propane sultone (1,3-PS), propyl propionate (PP), ethyl propionate (EP), fluoroethylene carbonate (FEC), propylene carbonate (PC), and ethylene carbonate (EC). Among them, based on the mass of the electrolyte, the mass ratio of LiPF 6 was 12.5%, the mass ratio of 1,3-PS was 3%, the mass ratio of PP was 45%, the mass ratio of EP was 15%, the mass ratio of FEC was 10%, and the balance was PC and EC. The mass ratio of PC and EC was 1:1.

[0073] Preparation of the separator: A PE composite film was used as the separator.

[0074] Preparation of the lithium-ion battery: After welding the tabs to the positive electrode plate and the negative electrode plate respectively, the positive electrode plate, the separator, and the negative electrode plate were stacked in sequence, with the separator placed between the positive electrode and the negative electrode to play a role in isolation. The electrode assembly was obtained by winding. The electrode assembly was placed in an aluminum-plastic film packaging bag, dried at 80°C, and then injected with the electrolyte. After vacuum packaging, standing, forming, shaping, and capacity testing processes, a soft-pack lithium-ion battery was obtained.

[0075] Examples 1-2 to 1-17

[0076] The differences between Examples 1-2 to 1-17 and Example 1-1 were that the mass ratio of carbon nanotubes to phenol and the mass concentration of ammonia water were changed, and the remaining steps were the same as those in Example 1-1. The specific preparation parameters can be referred to in Tables 1 and 2.

[0077] Examples 2-1 to 2-9

[0078] Examples 2-1 to 2-9 are different from Example 1-2 in that the diameter ratio of carbon nanotubes or the average circularity of silicon-carbon materials is changed, and the remaining steps are the same as those in Example 1-2. Specific preparation parameters can be referred to in Table 3.

[0079] Examples 3-1 to 3-8

[0080] Examples 3-1 to 3-8 are different from Example 1-14 in that the components and mass ratios of the electrolyte in the lithium-ion battery are adjusted. Except for adjusting the parameters of the relevant electrolyte components according to Table 4, the rest are the same as those in Example 1-14.

[0081] Comparative Examples 1 to 2

[0082] Comparative Examples 1 to 2 are different from Example 1-1 in that the mass ratio of carbon nanotubes to phenol and the mass concentration of ammonia water are changed, and the remaining steps are the same as those in Example 1-1. Specific preparation parameters can be referred to in Tables 1 and 2.

[0083] The silicon-carbon materials prepared in each example and comparative example and the assembled lithium-ion batteries are tested.

[0084] Performance tests of lithium-ion batteries

[0085] (1) Cycle performance test

[0086] Place the lithium-ion battery in an incubator at 25°C, charge it at a constant current of 1.5C to 4.4V, perform constant voltage charging at 4.4V until 0.05C, and then discharge it at a constant current of 1.0C to 3.0V. This is the first charge-discharge cycle. Conduct charge and discharge cycle tests according to the above steps, and take the ratio of the discharge capacity of each step to the initial capacity to obtain the discharge capacity retention rate of each step. Record the number of cycles of the lithium-ion battery at 25°C until the discharge capacity retention rate reaches 80%. The number of cycles of the lithium-ion battery at 25°C until the discharge capacity retention rate reaches 80% is used to characterize the cycle performance of the lithium-ion battery. The higher the number of cycles of the lithium-ion battery at 25°C until the discharge capacity retention rate reaches 80%, the better the cycle performance of the lithium-ion battery.

[0087] Capacity retention rate = remaining discharge capacity / initial discharge capacity × 100%.

[0088] (2) Discharge rate performance test

[0089] Under the condition of 25°C, charge the lithium-ion battery at 0.2C to 4.53V, and then discharge it at 0.2C to 3.0V. Record the discharge capacity C at this time 1 ; After standing for 5 minutes, charge it at 0.2C to 4.53V and discharge it at 0.5C to 3.0V. Record the charge capacity C at this time 2; After standing for 5 minutes, charge at 0.2C to 4.53V and discharge at 1.0C to 3.0V, and record the charging capacity C at this time 3 ; After standing for 5 minutes, charge at 0.2C to 4.53V and discharge at 2.0C to 3.0V, and record the charging capacity C at this time 4 。

[0090] Capacity retention rate at 0.5C (%) = C 2 / C 1 × 100%.

[0091] Capacity retention rate at 1C (%) = C 3 / C 1 × 100%.

[0092] Capacity retention rate at 2C (%) = C 4 / C 1 × 100%.

[0093] The rate performance of lithium-ion batteries is characterized by the ratio of the discharge capacity at different rates to the discharge capacity at 0.2C. The larger the ratio of the discharge capacity at different rates to the discharge capacity at 0.2C, the better the rate performance of the lithium-ion battery.

[0094] Test method for silicon-carbon materials:

[0095] (1) Test method for thermogravimetric analysis curve:

[0096] When performing thermogravimetric curve (TGA) testing in an air atmosphere, first, a silicon-carbon material sample needs to be prepared. Usually, the silicon-carbon material sample is ground into fine powder or cut into small pieces to ensure uniformity. Then, place the silicon-carbon material sample on the sample pan of the thermogravimetric analyzer and ensure good contact. Next, set the test parameters, including the heating rate (usually 5 - 20 °C / min), the temperature range (usually from room temperature to 1000 °C), and the flow rate of the air atmosphere (usually 50 - 100 mL / min). After starting the test, the thermogravimetric analyzer will record the curve of the sample's mass change with temperature, thereby obtaining the thermogravimetric analysis curve.

[0097] (2) Test method for the diameter ratio of carbon nanotubes:

[0098] First, disperse the carbon nanotubes in an appropriate solvent to ensure uniform dispersion and avoid agglomeration. Then, use a transmission electron microscope (TEM) to observe the structure of the carbon nanotubes and take high-resolution images. Next, measure the outer diameter and inner diameter of the carbon nanotubes through image processing software and calculate the diameter ratio (outer diameter / inner diameter). Finally, statistically analyze the diameter ratios of more than 10 carbon nanotubes to obtain the average value.

[0099] (3) Test method for the average circularity of silicon-carbon materials:

[0100] The particle samples of the negative electrode active material were observed using a ZEISS-SEM (sigma-02-33) scanning electron microscope. Twenty particles of the silicon-carbon material were randomly selected, and their perimeter equivalent diameters and area equivalent diameters were calculated respectively. The sphericity of each silicon-carbon material = perimeter equivalent diameter / area equivalent diameter. The arithmetic mean of the sphericities of the 20 silicon-carbon materials was calculated as the average circularity of the silicon-carbon material.

[0101] (4) Test method for the elastic modulus of the silicon-carbon material:

[0102] Using a Shimadzu single-particle crushing instrument (FLAT50), ten silicon-carbon material particles were randomly selected for crushing force testing, with a minimum / maximum pressure of 0.02 / 10 mN; according to the deformation amount and approximate contact area corresponding to the maximum pressure, and using Hooke's law and the pressure formula, the elastic modulus of each silicon-carbon material particle was calculated, and the arithmetic mean was obtained as the elastic modulus of the silicon-carbon material.

[0103] (5) Test method for measuring |S1 - S2| of the silicon-carbon material:

[0104] The silicon-carbon material particles containing carbon nanotubes were thinned by ion milling to form a thin slice with a thickness of about 80 nanometers. The prepared sample was placed on the sample stage of the TEM. In the high-resolution TEM mode, the distribution of carbon nanotubes inside the particles was observed and images were collected. The collected images were processed using image analysis software. Randomly select a cross-section including a first test area and a second test area with an area of 1000 nm 2 . Since the carbon nanotubes have obvious TEM lattice fringes, and the other regions of the carbon skeleton except the regions containing carbon nanotubes and the silicon particles are amorphous, the areas of the regions with obvious lattice fringes in the first test area and the second test area were measured, so as to obtain the proportion of the distribution area of carbon nanotubes in the two test areas S1 and S2, and the value of |S1 - S2| can be calculated.

[0105] (6) Test method for the powder conductivity of the silicon-carbon material:

[0106] The powder conductivity of the silicon-carbon material was tested using a conductivity tester (instrument model: Suzhou Jingge Electronics ST-2255A). Take 5 g of the silicon-carbon material powder sample, press it into a sample using an electronic press, apply pressure up to 5000 kg ± 2 kg, and maintain it for 20 s to obtain the test sample. Place the above test sample between the electrodes of the conductivity tester, and obtain the resistance R (unit: Ω) through the voltage U and current I at both ends. The height of the test sample is h (unit: cm), and the area of the test sample S = 3.14 cm 2 . According to the formula powder conductivity δ = h / (S × R), the powder conductivity of the silicon-carbon material was calculated, with the unit of S / cm.

[0107] Table 1

[0108]

[0109]

[0110] Table 2

[0111]

[0112] Combining Table 1 and Table 2, compared with the comparative examples, in Examples 1-1 to 1-17, during the preparation of the silicon-carbon material, by changing the mass ratio of carbon nanotubes to phenol and the mass concentration of ammonia water, when the intensity ratio of the first thermal weight loss peak to the second thermal weight loss peak of the silicon-carbon material satisfies a specific range, the cycle performance and rate performance of the silicon-carbon material can be improved.

[0113] And during the preparation of the silicon-carbon material, when the powder conductivity and elastic modulus of the silicon-carbon material are respectively within specific ranges, the cycle performance and expansion performance of the silicon-carbon material can be further improved.

[0114] Table 3

[0115]

[0116] In Table 3, during the preparation of the silicon-carbon material, by changing the tube diameter ratio of carbon nanotubes or the average circularity of the silicon-carbon material, when the tube diameter ratio of carbon nanotubes or the average circularity of the silicon-carbon material respectively satisfies specific ranges, the cycle performance and expansion performance of the silicon-carbon material can be further improved.

[0117] Table 4

[0118]

[0119]

[0120] Combining Table 4, in a lithium-ion battery, by changing the components of the electrolyte and the mass proportion of the lithium salt additive, when the electrolyte contains ethyl propionate and propyl propionate and within a suitable mass proportion range, and the components and mass proportion of the sulfur-oxygen double bond compound are maintained within specific ranges, the cycle performance and expansion performance of the silicon-carbon material can both be further improved.

[0121] The above-disclosed is only the preferred embodiment of the present application, and of course it cannot be used to limit the present application. Therefore, equivalent changes made according to the present application still fall within the scope covered by the present application.

Claims

1. A silicon-carbon material, characterized in that: It includes a carbon skeleton and silicon particles, wherein the carbon skeleton includes amorphous carbon and carbon nanotubes. In the thermogravimetric analysis curve of the silicon-carbon material, it has a first thermal gravimetric peak in the temperature range of 500°C to 800°C, and a second thermal gravimetric peak in the temperature range of 800°C to 1000°C, and the ratio of the intensity of the first thermal gravimetric peak to the intensity of the second thermal gravimetric peak is R, 10≤R≤1000.

2. The silicon-carbon material according to claim 1, characterized in that 15≤R≤850。 3. The silicon-carbon material according to claim 1, characterized in that The silicon-carbon material satisfies at least one of the following conditions: (1) The diameter ratio of the carbon nanotubes is L, 1.2≤L≤2; (2) The average circularity of the silicon-carbon material is 0.5 to 0.95; (3) The powder conductivity of the silicon-carbon material is 0.1 S / cm to 10 S / cm; (4) The elastic modulus of the silicon-carbon material is 50 GPa to 200 GPa.

4. The silicon-carbon material according to any one of claims 1 to 3, characterized in that The cross section of the silicon carbon material includes an area of ​​1000nm 2 A first test area and a second test area, the first test area and the second test area do not overlap, based on the area of ​​the first test area, the area of ​​the carbon nanotubes accounts for S1%, based on the area of ​​the second test area, the area of ​​the carbon nanotubes accounts for S2%, |S1-S2|≤0.

067.

5. A method for preparing a silicon-carbon material according to any one of claims 1 to 4, characterized in that: The following steps are involved: S1. Stirring carbon nanotubes, formaldehyde, phenol, and an alkaline catalyst to obtain a suspension, wherein the mass ratio of the carbon nanotubes to the phenol is (0.03-3):100; S2. The suspension is filtered and dried to obtain a solid product; S 3. sequentially carbonizing and activating the solid product to obtain porous carbon; S4. Using chemical vapor deposition, silicon is deposited and carbon is coated on the porous carbon to obtain the silicon-carbon material.

6. A secondary battery comprising a negative electrode sheet, a positive electrode sheet and an electrolyte, characterized in that: The negative electrode plate comprises the silicon-carbon material according to any one of claims 1 to 4 or the silicon-carbon material obtained by the preparation method according to claim 5.

7. The secondary battery according to claim 6, characterized in that: The electrolyte includes ethyl propionate and propyl propionate. Based on the mass of the electrolyte, the mass proportion of ethyl propionate is A1, the mass proportion of propyl propionate is A2, 2≤A2 / A1≤6, preferably, 3≤A2 / A1≤5.

8. The secondary battery according to claim 6 or 7, characterized in that: The electrolyte includes a sulfur-oxygen double bond compound, and the sulfur-oxygen double bond compound is selected from at least one of methylene methanedisulfonate, 1,3-propane sultone, 1,3-propane sultone, 2,4-butane sultone, or vinyl sulfate. 9 . The secondary battery according to claim 8 , wherein the mass percentage of the sulfur-oxygen double bond-containing compound is P% based on the mass of the electrolyte, and 1≤P≤5.

10. An electronic device, characterized in that: The invention comprises the secondary battery according to any one of claims 6 to 9.

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